Gas turbine engines, such as those which power aircraft, employ a compressor to compress air that is drawn into the engine and a turbine to capture energy associated with the combustion of a fuel-air mixture. The engine is housed within a structure that is referred to as a nacelle. The nacelle is supported by a pylon that connects to a structure (e.g., a wing) of an aircraft.
One or more ducts may be associated with the nacelle or pylon. For example, a so-called pre-cooler duct may be coupled to the nacelle/pylon. The pre-cooler duct is used to route a fluid, such as air. For example, the air is received at an inlet and is conveyed by the duct to a location downstream of the inlet, where the air is frequently conditioned for other uses (e.g., aircraft cabin cooling).
As a result of the configuration described above, one or more acoustic tones may be generated when there is little or no flow being ingested by the inlet. For example, a fundamental tone/frequency and harmonics thereof may be generated as a result of a coupling between turbulence in a separated flow (e.g., a shear layer) over the duct intake (e.g., the inlet) and the duct response. At a minimum, the tone and its harmonics represent a nuisance or an observable source of noise. However, if the coupling is sufficiently large (e.g., if the coupling is proximate to a resonant condition), the tonal output may result in degraded performance or functionality of components associated with the aircraft.